1. Field of the Invention
The present invention relates to a process for producing an electrode substrate having a light shielding film, suitable for use in liquid-crystal devices such as a liquid-crystal display device and a liquid crystal-optical shutter device. More particularly, the present invention relates to a process for producing an electrode substrate with a light shielding film which is placed between pixels of a liquid crystal display device or in areas other than a window of a liquid crystal-optical shutter device.
2. Description of the Related Art
Hitherto, liquid crystal devices have been known which utilize twisted nematic liquid crystal of the type disclosed in "Voltage-Dependant Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, in "Applied Physics Letters" Vol. 18. No. 4, pp 127-128.
In this type of liquid crystal device, the number of pixels is undesirably limited due to the fact that the twisted nematic liquid crystal tends to allow crosstalk when driven by a multiplexing process with a matrix-type electrode structure having a high pixel density.
A display device also has been known in which switching elements composed of thin-film transistors are connected to the respective pixels so as to enable switching of the pixels independently. This type of display device, however, requires quite a complicated process for forming such thin-film transistors on a substrate. In addition, it is difficult to produce a display device of this type with a large display area.
In order to overcome this problem, a ferroelectric liquid crystal device has been proposed by Clark et al. in the specification of U.S. Pat. No. 4,367,924.
FIG. 3 schematically shows an example of a liquid crystal cell for the purpose of illustration of an operation of the ferroelectric liquid crystal. Referring to this Figure, numerals 21a and 21b show substrates covered by transparent electrodes composed of transparent conductive thin films such as In.sub.2 O.sub.3, SnO.sub.2 or ITO (Indium-Tin Oxide). The space between these substrates 21a, 21b is filled with a liquid crystal of SmC* phase or SmH* phase oriented such that a plurality of liquid crystal molecule layers 22 are perpendicular to the substrates. Thick lines 23 in this Figure show liquid crystal molecules. The liquid crystal molecule 23 has a bipolar moment (P.perp.) 24 in the direction perpendicular to the molecule. When a voltage exceeding a predetermined threshold value is applied between the electrodes on the substrates 21a and 21b, the spiral arrangement of the liquid crystal molecules 23 is loosened so that the orientation of the liquid crystal molecules 23 can be changed such that the bipolar moments (P.perp.) 24 of all the molecules are aligned in the direction of the electric field. The liquid crystal molecule has an elongated form so as to exhibit a refractive anisotropy, i.e., different refractive index values in the directions of longer and shorter axes. It is therefore easy to understand that a liquid crystal optical modulation device, which varies its optical characteristics depending on the polarity of the voltage applied, can be obtained by disposing polarizers on the outer sides of both glass substrates in a cross-conical relationship to each other.
Liquid crystal cells suitably used in the ferroelectric liquid crystal device can be very thin, e.g., 10.mu. or less. As the thickness becomes smaller, the spiral arrangement of the liquid crystal molecules becomes loose even in the absence of an electric field, with the result that the liquid crystal molecules are oriented to direct their bipolar moments upward (Pa) as denoted by 34a or downward (Pb) as denoted by 34b, as will be seen from FIG. 4. When either an electric field Ea, stronger than a predetermined threshold, or an electric field Eb, having the opposite polarity, is applied to this cell, the bipolar moments of the liquid crystal molecules are oriented upward as 34a or downward as 34b depending on the vector of the electric field Ea or Eb, whereby the liquid crystal molecules are set either to a first stable state 33a or a second stable state 33b.
As explained before, the ferroelectric liquid crystal device as an optical modulating element offers two major advantages: namely, an extremely high response speed and bi-stability of liquid crystal molecule orientation.
The second advantage, namely, the bi-stability of the ferroelectric liquid crystal device, will be described in more detail with reference to FIG. 4. The liquid crystal molecules are oriented to the first stable state 33a when an electric field Ea is applied. This state is stably maintained even after extinction of the electric field. When the electric field Eb of the opposite polarity is applied, the liquid crystal molecules are oriented to the second stable state 33b. This state also is maintained even after extinction of the electric field Eb. In addition, each stable state is maintained without being inverted even by application of an electric field Ea or Eb of the opposite polarity, unless the magnitude of the electric field applied exceeds the predetermined threshold. In order to realize the high response speed and bi-stable nature as described, the cell thickness should be made as small as possible.
In order that the ferroelectric liquid crystal device exhibits the expected driving characteristic, it is necessary that the molecules of the ferroelectric liquid crystal, interposed between the pair of parallel substrates are arranged to allow an easy switching between the two stable states. For instance, in the case of a ferroelectric liquid crystal having chiral smectic phase, it is necessary that a mono-domain is formed in which the liquid crystal molecule layers of chiral smectic phase are perpendicular to the substrate surfaces and, hence, the axes of the liquid crystal molecules are substantially parallel to the substrate surfaces. However, a liquid crystal orientation having a mono-domain structure could not be satisfactorily obtained in the conventional ferroelectric liquid crystal device, making it impossible to attain a desired characteristic of the liquid crystal device.
In the production of a panel making use of such liquid crystals, it is conventional to form a light-shielding film between pixels, for the purpose of enhancing contrast. Practically, such a light-shielding film is formed by a process having the steps of spin-coating a photosensitive anaerobic colored resin and covering the resin with a polyvinyl film as an oxygen shielding film, followed by exposure and development.
It has also been attempted to enhance the contrast by forming a metal light-shielding film.
These known processes of producing electrode substrates, however, suffer from the following disadvantages. In an ordinary ferroelectric chiral smectic liquid crystal panel, a step of about 4000 .ANG. is formed due to the presence of a laminate composed of an ITO transparent conductive film of about 2500 .ANG. thick and a molybdenum metal film of about 1500 .ANG. thick. Therefore, the light-shielding film which is to be formed between adjacent islands of the laminate has to have a very small thickness of about 4000 .ANG.. Actually, however, it is impossible to apply a resin by spin-coating in a thickness less than 1 .mu.m, so that thin light-shielding film having such a very small thickness of 4000 .ANG. or so is materially not obtainable.
It has been attempted to spin-coat the resin of the light-shielding film material with a thickness of 4000 .ANG. or so by adjusting the density of the resin. It has been reported that, when such a process is used, the thickness of the light-shielding film is more than 50% greater at the central region of the substrate than at other regions. Thus, the light-shielding film produces a step of large height, making it impossible to obtain a chiral smectic liquid crystal orientation having a good bi-stable orientation.
In order to reduce the height of the step at the surface of the electrode substrate, it has also been proposed to form a light-shielding film from a metal having an excellent light-shielding characteristic with the same thickness as the thickness of the laminate composed of indium oxide film and molybdenum film, e.g., 4000 .ANG.. This type of liquid crystal device exhibits excellent state of orientation of bi-stable chiral smectic liquid crystal due to high degree of flatness, but suffers from a problem in that the driving of the liquid crystal is failed due to short-circuiting between the light-shielding metals which are provided on upper and lower substrates and which serve as electrodes.